Apparatus of the aforementioned type is known above all under the term material bed roll mill (cf. in this connection, for example, Walter H. Duda, Cement-Data-Book, Volume 1, 3rd Edition 1985, pages 255 to 257). In these known material bed roll mills two comminuting rolls which are driven so as to rotate in opposite directions and are pressed against one another with a high pressure are provided as comminuting tools in which the length:diameter ratio is generally in the range from 0.3 to 1. In this case one comminuting roll, the so-called fixed roll, is mounted so as to be stationary, whilst the second roll, the so-called floating roll, is mounted so as to be movable relative to the first roll against the force of a spring. In the unloaded starting position a working gap distance is maintained between the two comminuting rolls with the aid of spacers, whereby the working surfaces of the rolls which cooperate in the comminuting work do not touch. A somewhat concave working surface develops on the outer peripheral surface of the rolls as a result of the corresponding depth of wear. The maximum depth of wear of these two working surfaces which are directed towards one another plus the so-called zero gap distance, i.e. the minimum peripheral distance between the two rolls when the spacers are juxtaposed, form the aforementioned working gap distance in the unloaded starting position (juxtaposed spacers).
In material bed comminution of brittle material for grinding, both individual grain comminution and also material bed comminution take place in the grinding gap between the two grinding rolls, in which case the material for grinding which is leaving the grinding gap, that is to say the comminuted material, is substantially pressed to form agglomerates, so-called "scabs", which can then be disagglomerated and/or further comminuted.
In efficient material comminution of the aforementioned type, the output or throughput rate, the grinding force, the roll mounting and the necessary grinding gap size (scab thickness) as well as the axial length and the diameter of the rolls play an essential part, the roll length generally being a resultant figure produced from the necessary output. In the choice of roll diameter and the length:diameter ratio there are various dependences which play a part. Thus in view of the relatively small proportion in percentage terms or the material bed which is pressed less at the roll edges it has proved favourable to choose relatively great roll lengths and thus relatively great length:diameter ratios. On the other hand, with smaller roll lengths or greater roll diameters, and thus smaller length:diameter ratios, it has proved favourable that the danger of skewing of the rolls is less, an even material feed can take place more easily and also the roll wear is less with thick scabs than with thin scabs which form with smaller roll diameters, since scab thickness and roll diameter are directly proportional. Since roll length and roll diameter determine as product the output of the apparatus, the length:diameter ratio basically has no influence of the output so long as the product of length and diameter remains the same.
However, in all these known types of construction particular difficulties are caused by the frequently high wear on the outer circumferential surfaces and thus on the working surfaces of the rolls. Even when the comminuting tools which are constructed as rolls have a cladding layer made from a suitable hard material on their outer circumference, during the comminuting operation the aforementioned somewhat concave working surface forms with a more or less great depth of wear in the region of the centre of the length of the roll, because in this central roll region the feed material is more easily drawn in than in the region of the edges of the rolls and because the pressure in the material bed is greatest in the region of the centre of the rolls.
Therefore in these known material bed roll mills it is necessary after a certain operating time to recondition the rolls, i.e. to re-turn or re-grind them or also to build them up by deposit welding of material at particular deep wear locations in order to equalise the resulting wear profile on the working surface. These various re-machining operations are not only time-consuming and costly but in many cases they also reduce the availability of the entire comminuting apparatus in a manner which is no longer acceptable. Moreover this is also a reason why these known material bed roll mills or material bed comminuting apparatus are accepted only hesitantly or not at all in industries in which particularly aggressive materials are comminuted.
The object of the invention, therefore, is to create a material bed comminuting apparatus which has a good output and is distinguished by a relatively uniform wear on the working surfaces of the the comminuting tools thereof and largely avoids the need for re-machining of these working surfaces.